1. Field of the Invention
This invention relates to sealed primary galvanic cells having alkaline electrolytes. In particular, it relates to such cells having a barrier and a barrier protecting separator between the electrodes, the barrier protecting separator containing a pore restricting material.
2. Description of the Prior Art
High energy density electrochemical systems such as silver-zinc, mercury-zinc, nickel-cadmium, silver-cadmium, manganese-zinc, and mercury-cadmium are well known in the art and used in alkaline electrolyte cells where high energy density is required. Such high energy density batteries or cells are generally battery systems which have a substantially higher energy per unit of weight than conventional, e.g. lead storage batteries. Such high energy density batteries or cells can for example develop from 100-140 watt hours of energy per pound. These batteries or cells have numerous applications such as in portable tools and appliances, televisions, radios, hearing aids, and electric and electronic watches.
In batteries or cells of this type, the barrier employed performs the function of retaining electrolyte, e.g. potassium hydroxide, separating the electrodes, and deterring migration of electrode ions such as silver ions or growth of dendrite crystals of electrode ions such as zinc ions, which can short circuit the battery. The use of organic barriers in this capacity is well known in the art, however, such use is attended by certain problems. For example, such organic barriers may not be chemically stable, particularly at temperatures above 50.degree. C; they may tend to swell excessively or otherwise degrade. Additionally, organics are not inert to oxidizing agents in caustic solutions; can be easily punctured by dendritic growth and may not effectively prevent ion migration which will result in self-discharging of the cell. The art is further complicated by the fact that barrier systems and other design parameters which result in a battery with high rate capability, typically are suboptimal for shelf life characteristics. That is, high rate capability alkaline batteries typically have poorer shelf life than low rate capability alkaline batteries, and conversely, long shelf life batteries typically do not have high rate capability. As a result of this trade-off, separator systems are sought which strive to optimally compromise the rate capability/shelf life characteristics of a given battery system.
An organic barrier for primary alkaline galvanic cells is known comprising an organic substrate at least one side of which has adhered thereto a mixture consisting essentially of a binder material and an inorganic material wherein the inorganic material is selected from the group which consists of titanium dioxide, zirconium dioxide, aluminum sulfate, aluminum chloride, aluminum oxide, barium chloride and chromium chloride and wherein the binder material is selected from the group which consists of magnesium hydroxide, carboxy methyl cellulose, guar gum, carbopol and mixtures thereof.
A novel battery barrier protecting separator has now been discovered which will greatly improve the shelf life characteristics of high rate battery systems, while at the same time maintaining the high rate capability. The separator will also permit the redesign of low rate systems to achieve high rate capability while maintaining good shelf life characteristics.